Polyorganosiloxane has excellent heat resistance, weatherability and surface-modifying function, and therefore it is used for semiconductor insulating protective films, flame retardants and coating material additives. For example, a coating agent prepared by blending an organic polymer with polyorganosiloxane can provide the surface of a material coated with the agent with functions such as water repellency. The representative examples of the organic polymer are acryl resins, polyurethanes and alkid resins. In general, however, these polymers do not have a good compatibility with polyorganosiloxane. Accordingly, there have been the problems that the coating agent is liable to become cloudy by blending polyorganosiloxane, and that a coating film obtained from the coating agent is liable to be whitened. That is, an addition amount of polyorganosiloxane has been restricted.
It has so far been known that an introduction of a polysiloxane structure into principal chain and/or side chain of an organic polymer makes it possible to improve the characteristics of the polymer such as heat resistance, water repellency and weatherability. Disclosed in, for example, the patent document 1 is a process in which a polysiloxane-containing polymer is radically copolymerized with other addition-polymerizable monomers to thereby produce a polysiloxane graft copolymer having a polysiloxane structure in a side chain. Polysilsesquioxane having a structure in which 1.5 oxygen atom is bonded to one silicon atom is disclosed in the patent document 2. It is described in the document that a polysilsesquioxane derivative having a polymerizable unsaturated bond and two or more functional groups such as hydroxyl group and alkoxy group is copolymerized with other addition-polymerizable monomers to thereby obtain a vinyl polymer into which a siloxane side chain is introduced. All of them are considered to be excellent in heat resistance, water repellency and weatherability as compared with the homopolymers of the other addition-polymerizable monomers.
It has so far been tried to enhance the content of a polyorganosiloxane structure in an organic polymer for the purpose of improving the characteristics as described above. However, in the foregoing organic polymer having a polyorganosiloxane structure, such a high improving effect as expected has not been provided to the characteristics such as heat resistance, water repellency, weatherability and electric insulating property. Accordingly, strongly desired is a polyorganosiloxane having a structure which further elevates characteristics such as heat resistance, water repellency and weatherability in the organic polymer.    Patent document 1: JP S60-231720 A/1985    Patent document 2: JP S62-275132 A/1987
An object of the present invention is to provide a useful polysilsesquioxane derivative in order to solve the problems described above, and another object is to provide a novel polymer obtained by using the derivative. Still another object is to provide a coating agent, a plastic substrate and an optical material each obtained by using the polymer.